Structural monitoring sensor system

ABSTRACT

The present invention is directed to a system for continuous physical integrity monitoring of large civil structures such as bridges and high-rise buildings . . . wherein the relevant sensor data stream is generated continuously and transmitted to the data gathering location without the need for an incoming triggering signal of any kind; i.e., it is a one way transmission system. Specifically, it is a concept for an interlinked multi-parameter Early Warning Sensor system with a full time data management capability for structures. The invention is also directed to both the system construction, with its communication capability, and also designs of specific sensors applicable to the system as a whole. As a practical example of application of the present invention to a structure, the description in this application is directed primarily towards system applications for bridge integrity early warning systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of prior U.S. application Ser. No.09/772,182, filed Jan. 29, 2001, now U.S. Pat. No. 6,487,914, which is adivision of U.S. application Ser. No. 09/481,289, filed Jan. 11, 2000,now U.S. Pat. No. 6,181,841, which is a division of U.S. applicationSer. No. 09/097,268, filed on Jun. 15, 1998, now U.S. Pat. No.6,012,337, which is a continuation of application No. PCT/US96/20015,filed Dec. 13, 1996, which designated the United States of America andclaimed the benefit under 35 U.S.C. 119(e) of U.S. ProvisionalApplication No. 60/008,687, filed Dec. 15, 1995, and U.S. ProvisionalApplication No. 60/011,164, filed Feb. 5, 1996.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention has been created without the sponsorship or funding ofany federally sponsored research or development program.

BACKGROUND OF THE INVENTION

The present invention is directed to a system for continuous physicalintegrity monitoring of large civil structures such as bridges andhigh-rise buildings . . . wherein the relevant sensor data stream isgenerated continuously and transmitted to the data gathering locationwithout the need for an incoming triggering signal of any kind;. i.e.,it is a one way transmission system. Specifically, it is a concept foran interlinked multi-parameter Early Warning Sensor system with a fulltime data management capability for structures. The invention is alsodirected to both the system construction, with its communicationcapability, and also unique designs of specific sensors applicable tothe system as a whole. As a practical example of application of thepresent invention to a structure, the description in this application isdirected primarily towards system applications for bridge integrityearly warning systems. However it should be understood that the systemand its benefits may be applied to a wide range of physical structures.

The system of the present invention, as applied to bridges, is unique inits ability to address the four principal failure mechanisms orprecursors to failure most commonly associated with bridges. These are:

1. Catastrophic failure where some major structural defect progressesundetected to the point where some critical section of the bridgecollapses. This will be designated Slow Movement Failure.

2. Vibration-associated Failure where sporadic traffic loading creates avibration environment which can accelerate failure, such as fatigue, andalso be a diagnostic tool useful in predicting failure. This isdesignated Rapid Movement Failure.

3. Corrosion-induced Failure where the steady winter applications ofsalt eventually permeate the concrete to the depth of the rebars whichbegin to corrode. This weakens the rebars and also causes the concreteto spall off the bars. It also weakens the concrete. This is designatedCorrosion Failure.

4. Low temperature-induced failure where a freezing road bed can lead tofrost formation and resultant pot-hole development. Pot-holes canexaggerate the stress on the entire bridge structure through vehicularimpact. This is designated Temperature Related Failure, and it isaddressed through Temperature Sensing and Pot-Hole Sensing.

The present invention encompasses to major aspect of novelty. The firstaspect is a harness which is attached permanently to a structure. Thisharness permits an array of interconnected transducers to be deployed atspecific sites on the structure for specific sensing applications. Italso provides the sensors with a common electro-optic interface whichmay be linked with a remote communication system . . . by a one-way datatransmission system which does not require an incoming signal stimulusto trigger the sensor data download.

The second aspect relates to various types of the sensors which may beattached to this harness. Both analog and digital sensor types aredescribed, and specific embodiments for corrosion monitoring, potholemonitoring, vibration monitoring and temperature monitoring are includedin the present disclosure, as well as traffic flow, scour, bridge deckdeflection, cross-wind velocity, temperature, fire, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic general view of an optical monitoring system towhich the present invention is applied;

FIG. 2 is a diagrammatic view of a portion of the optical monitoringsystem which includes a feature of the present invention for determiningdirection of movement;

FIG. 3 is a diagrammatic view of a conventional encoder pattern for anoptical sensor which forms part of the optical monitoring system;

FIG. 4 is a chart showing relative displacement of the reticle and maskelements which form part of the optical sensor, plotted againstreflected light intensity for the sensor encoder pattern of FIG. 3;

FIG. 5 is a chart showing the digitizing of the reflected light signalsof the chart of FIG. 4;

FIG. 6 is a diagrammatic view of a first embodiment of an encoder gridgeometry for the reticle and mask of the optical sensor of the presentinvention;

FIG. 7 is a chart showing relative displacement of the grid elements ofthe optical sensor plotted against reflected light for the encoderpattern of FIG. 6;

FIG. 8 is a chart showing the digitizing of the reflected light signalsof the chart of FIG. 7;

FIGS. 9 and 10 are diagrammatic views of an encoder pattern for thereticle and mask elements of the optical sensor of the presentinvention, showing a modification for detecting direction of relativemovement of the reticle and mask by means of quadrature. FIG. 9 is themask and FIG. 10 is the reticle with two tracks having the quadrature90° offset;

FIGS. 11 and 13 are diagrammatic views of a further modified encoderpattern for reticle and mask of the optical sensor of the presentinvention;

FIGS. 12 and 14 are charts showing the optical power pattern resultingfrom the relative displacement of the reticle and mask of the opticalsensor plotted against reflected light intensity for the encoder patternof FIGS. 11 and 13 respectively;

FIGS. 15, 16 and 17 are diagrammatic views of a further refinement ofthe modified encoder patterns of FIGS. 11 and 13 which permitsdeliberate alteration of the optical power patterns of FIGS. 12 and 14;

FIG. 18 is a chart showing the relative displacement of the reticle andmask of the refinement of the modified encoder patterns of FIGS. 15-17;

FIG. 19 is a front elevational view of a tensioner/sensor assembly formonitoring a structure;

FIG. 19A is a diagrammatic view of the sensor portion of thetensioner/sensor assembly of FIG. 19;

FIG. 20 is diagrammatic view of a sensor as applied to a bridge formonitoring deflection;

FIG. 20A is an enlarged view;

FIG. 21 is a diagrammatic view illustrating a combination deckdeflection and pier tilt monitoring system with the use of sensors;

FIG. 22 is a diagrammatic view illustrating a sensor system fordetecting scour at the base of a bridge pier;

FIGS. 22A and 22B are diagrammatic illustrations of a tilt meteremploying the principles of the present invention;

FIG. 23 is a diagrammatic view of a modified monitoring system fordetecting scour at the pier portion of a bridge;

FIG. 24 is a diagrammatic view of an application of the monitoringsystem of the present invention for monitoring bridge temperature;

FIG. 25 is a diagrammatic view of a monitoring system of the presentinvention for monitoring wind velocity;

FIG. 26 is a diagrammatic view of a monitoring system of the presentinvention for detecting potholes in the deck portion of a bridge;

FIG. 27 is a graph illustrating the application of the pothole detectionsystem of FIG. 26 for determining a repair threshold value;

FIG. 27A is a diagrammatic view of a modified application of the potholedetection system of FIG. 26;

FIG. 27B is a flow chart of the pothole sensing algorithm for thepothole monitoring systems of FIGS. 26 and 27A;

FIG. 28 is a diagrammatic illustration of the corrosion sequence of arebar for monitoring corrosion in concrete which expands over time;

FIG. 29 is a graph showing the corrosion over time sequence of the rebarof FIG. 28;

FIG. 30 is a diagrammatic view of a corrosion sequence for a rebar formonitoring corrosion in concrete which shortens over time;

FIG. 31 is a graph illustrating the corrosion over time sequence of therebar of FIG. 30;

FIG. 32 and 33 is a diagrammatic view of the first environment of acorrosion monitoring sensor for concrete employing rebars which shrinkover time;

FIGS. 34 and 35 are diagrammatic views of a second modification of acorrosion sensor for concrete having rebars which shrink over time;

FIGS. 36, 37, 38 and 39 are diagrammatic views of a third modificationof a corrosion monitoring sensor for concrete using rebars which shrinkover time;

FIGS. 40 and 41 are diagrammatic views of apparatus for monitoring thesurface hardness of concrete over time;

FIG. 42 is a top plan view of the reflective grid portion of a fatiguefuse embodying the principles of the present invention;

FIG. 43 is a top plan view of the transmissive mask portion of a fatiguefuse of the present invention;

FIG. 44 is a top plan view of the reflective grid and transmissive maskportions of FIGS. 42 and 43;

FIG. 45 is a side elevational view of a fatigue fuse of FIG. 44 shownapplied to a structure to be monitored;

FIG. 46 is a chart showing testing results for the fuse of FIGS. 44 and45;

FIG. 47 is a diagrammatic view of the monitoring system of the presentinvention as applied to monitoring temperature; and

FIG. 48 is a diagrammatic view of a modification of the monitoringsystem of the present invention for monitoring temperature.

DETAILED DESCRIPTION OF THE INVENTION OPTICAL MONITORING SYSTEM

The optical structural integrity monitoring system of the presentinvention includes a sensor interrogation harness which exploits asimple sensor differentiation technique known as Time DivisionMultiplexing, TDM. Since light travels through an optical fiber at afixed velocity, each sensor is attached to the pulsed laser source by adifferent length of fiber. Further, by also causing the sensors' outputto be reflected back down the same fiber to the photo-detector, thedifferential delay is precisely doubled.

Referring first to FIG. 1, the optical monitoring system of the presentinvention is generally indicated by the reference numeral 3 and includesa laser 2 which is capable of generating pulses of light 4 into one leg7 of a Y-coupler coupler 6. The other leg of the coupler is connected toa photo detector 14 which, in turn, is operatively connected tocircuitry 18. A cabled bundle of optical fibers 8, is connected to theY-coupler 6. A single optical fiber from the cable 8 is connected toeach of a plurality of optical sensors 12 located at strategic locationson the structure which is being monitored, in those instances where thedirection of motion of the sensor is unambiguous. Each sensor has an“on”, or reflecting condition and an ‘off’, or non-reflecting condition,to be described. Each light pulse from the laser 2 proceeds to thecables 20 and 22 via the coupler 6 to each of the sensors in the system.If a sensor is in its reflective condition, some tangible portion 10 ofthe light pulse will travel back down the same optical fiber and passthrough the Y-coupler 6 and on to the photo detector 14 via the cable 9.

The circuitry 18, of the photo-detector is programmed to clock thearrival, or non-arrival depending on the sensor's condition, in certaintime windows. These are known and programmed into the computer whichwill therefore know which sensor is responding in whatever mode,reflective (logical one), or non-reflective (logical zero). Because thelaser 2 is pulsing at a frequency of up to half a million cycles persecond, 0.5 MHZ, there is ample opportunity to capture the change fromdetectable signal to non-detectable without missing a step in thesequence.

Each optical sensor 12 is mounted on the structure to be monitored todetect the relative movement of a first element of the structurerelative to a second element of the structure along a first axis. Eachsensor comprises a probe 21 which is slidably mounted within a housing23. The probe contains a transmissive grid, or reticle. The housingcontains a reflective grid, or mask. The reticle moves longitudinallyrelative to the mask as the probe moves relative to the housing. Anoptical fiber from the fiber optic cable extends into the housing sothat the end of the optical fiber is at the reticle for transmitting apulse of light at a right angle to the reticle. Light passing throughthe transmissive areas of the reticle is reflected by the mask back tothe end of the optical fiber. Such a sensor is known as a reflectiveoptical sensor. The present invention is also applicable to atransmissive optical sensor which is similar to a reflective opticalsensor except that the reflective areas of the mask are transmissiveareas. Light from the optical fiber passes through the transmissiveareas of the reticle and mask and strikes the end of a second opticalfiber at the opposite side of the housing for transmission to the Ycoupler. The probe is fixed to a first element of the structure to bemonitored. The housing is fixed to the second element of the structureto be monitored.

The reticle and mask are located in separate spaced parallel planes. Themask is mounted in the encoder for movement relative to the reticle inaccordance with the relative movement between the first and secondelements of the structure to be monitored. The mask and the reticlefunction as an encoder for the light pulses received from the laser andreflected to the photo-detector 14. The reticle has a plurality ofevenly spaced light impervious surfaces. The areas between the lightimpervious surfaces are pervious to light. The pervious areas are theactive areas of the reticle and the light impervious areas are thepassive areas of the reticle. The mask has a plurality of evenly spaceduniform reflective surfaces which are considered the active areas of themask. The areas between the reflective surfaces are non-reflective andare considered the passive areas of the mask.

FIG. 2 shows a detail of the basic interrogation harness of FIG. 1having to do with “quadrature”, which allows the detection circuitry tobe able to determine the direction of relative movement of the elementsof the structure which are being monitored. The cable or bundle ofoptical fibers connected to the coupler 6 are divided into two groups ofoptical fibers, indicated by the reference numerals 20 and 22. A singleoptical fiber from each of the groups 20 and 22 is connected to eachsensor 12 at various strategic locations along the structure to bemonitored. As shown in FIG. 2, a single optical fiber 20 a from thebundle of fibers 20 is connected to sensor 12 a and a single fiber 22 afrom the bundle of optical fibers 22 is connected to sensor 12 a. Asingle optical fiber 20 b from the bundle of optical fibers 20 isconnected to sensor 12 b and a single optical fiber 22 b from theoptical fiber bundle 22 is connected to sensor 12 b. The bundle offibers, or cable 22 is configured to include an extra length, indicatedby the reference numeral 25, immediately adjacent the Y-coupler 6. Thisenables any sensor in the system which requires a dual fiber quadraturefeature may have a fiber selected, one from each of the two cables 20and 22, at a specific physical location. The quadrature feature isdescribed in greater detail hereinbelow in connection with FIGS. 9 and10. The cables are sufficiently different in length so that theirrespective output pulses will be distinguishable via a TDM protocol.

The encoder geometry of the reticle and mask of conventional sensorsemploy equal active and passive areas for the reticle and mask patternsas shown in FIG. 3, wherein each pattern, generally indicated by thereference numeral 16, has a plurality of active areas 15 which alternatewith a plurality of passive areas 17. The result of employing two equalpatterns, each having an equal active area to passive area ratio,creates a saw-tooth pattern output as shown in FIG. 4. In manystructural applications, it is important to be able to monitor smallrelative movements between two structural elements. Therefore, it isdesirable to have this “off” intervals of the sensor equal the “on”intervals as closely as possible. The problem is that it is necessary tobe able to locate the half-height intensity of the saw tooth 24, FIG. 4,if one is to divide the output into equal on and off intervals, see 26,FIG. 5. This is normally accomplished by the technique of measuring thefull peak height 28, FIG. 4, and conducting the mathematics in thecircuitry of the light detection logic circuit. In order to be able topower a multiplicity of different fiber-sensor systems from a singlesource and detecting their outputs at a single detector, the presentinvention provides a low cost and simple approach which accommodateswidely varying power level from sensor to sensor.

The problems associated with conventional prior art sensors is overcomeby the sensor 12 of the present invention. Each sensor 12 of the presentinvention includes a grid encoder design which differs substantiallyfrom those of conventional sensors. A first embodiment of applicant'sencoder grid design is shown in FIG. 6, wherein the pattern or geometryof the reticle and the mask, generally indicated by the referencenumeral 29, includes a plurality of equally spaced uniform active areas33. The areas between the active areas 33, indicated by the referencenumeral 31, are twice as wide as the active areas 33 along thelongitudinal axis of the grid or the first axis. The active areas 33 ofthe reticle are the light pervious areas and the active areas of themask are the reflective areas. This mask geometry automatically causesthe light output to blink on and off in equal proportions by theexpedient of keying off the baseline triggering signal level 30, FIG. 7,rather than the sensor-specific mid-height peak intensity. As shown inFIG. 7, the absolute peak height range 32, has little affect on this newbaseline triggering protocol, as seen by the output signal 36, FIG. 8.

Referring to FIGS. 9 and 10, there is shown a first modified grid designof the present invention, wherein the transmission grid, or reticle,generally indicated by the reference numeral 38, is identical to thegrid design of FIG. 6, while the reflective grid, or mask, generallyindicated by the reference numeral 40, has two identical portions,generally indicated by the reference numerals 40 a and 40 b, as shown inFIG. 10. The reticle 38 has active, or light pervious areas 35 andpassive, or light impervious areas 37. Each portion 40 a and 40 b of themask 40 has active, or reflective areas 39 and passive, ornon-reflective areas 41. The non-reflective areas 41 are twice as wideas the reflective areas 39. The second portion 40 b is offset from thefirst portion 40 a by half the distance of the width of an active areaalong the longitudinal axis of the mask. In this embodiment, two opticalfibers are employed as shown in FIG. 2. One optical fiber is alignedwith the portion 40 a and the other optical fiber is aligned with theportion 40 b. Each optical fiber has an end surface which is parallel tothe plane of the mask 40 for directing a pulse of light transversely ofthe active surfaces of the reticle and the mask. Directionality of therelative movement between the elements of the structure being monitoredis resolved by quadrature as provided by the offsetting of the offsetportions 40 a and 40 b. The variability in light directionality which isencountered in actual practice may require slight modification in the2:1 ratio to produce the equal ‘on’ and ‘off’ condition.

Referring to FIGS. 11-13, there is shown another modification of theencoder geometry of the present invention, generally indicated by thereference numeral 48. In the sensor geometry 48, the reticle and themask do not employ the same active area to passive area ratios.

The sensor geometry 48 includes a transmissive grid, or reticle 42 and areflective grid, or mask 43. The reticle 42 has a plurality of uniformlyspaced light impervious surfaces 45 which are the passive areas of thereticle. The areas between the surfaces 45, indicated by the referencenumeral 47, are light pervious and are the active areas of the reticle.The widths of the active and passive areas of the reticle 42 along thefirst, or central longitudinal axis of the sensor, is one-to-one. Thatis, the width of each light impervious surface 45 is equal in width toeach light pervious area 47 along the first axis. The mask 43 has aplurality of uniform, equally spaced reflective surfaces 49. The areasbetween the reflective surfaces 49, indicated by the reference numeral51, are non-reflective and represent the passive areas of the mask 43.Reflective surfaces 49 represent the active areas of the reflective mask43. The width of each passive area 51 is substantially larger than thewidth of each active area 49 along the first axis, or longitudinal axisof the sensor.

It has been found that threshold triggering will yield an approximationof the 1:1 digital switching with either 4:1 or 5:1 reticle:mask activearea ratios, see FIGS. 11 and 13. The corresponding signal thresholdswitching outputs are depicted in FIGS. 12 and 14, respectively. Thelateral mask 43, motion relative to the fixed reticle 42, in FIG.11 isshown as a descending sequence of zeros and ones against the sequentialnumber on the right hand side of FIG. 11. The reflective ratio in FIG.11 is 4:1 and a 5:1 ratio is shown in FIG. 13. The higher ratio willmore closely approach the desired signal equality at thresholdswitching, but at the expense of reflective active area and concomitantreflected signal strength. It is apparent from FIGS. 12 and 14 that the‘logical one’ periods resulting from threshold triggering 44 are longerthan the ‘logical zero’ periods 46, FIGS. 12 and 14.

This analysis was conducted under the assumption that thereticle-impinging light is orthogonal to the structure 48, FIG. 16.However, in actuality this is not the case. The light will emerge fromthe optical fiber at a range of angles, typically up to 25 degrees or soto the vertical 50, as shown in FIG. 16. This results in a decrease inthe effective area of the reflector patches, thus effectively reducingthe on cycle, and evening up the intervals between “logical ones” and“logical zeros” for the desire threshold switching protocol. This isillustrated as light rays 52, FIGS. 16 and 17 mathematically in FIG. 15.The precise relationship for the anticipated signal shift and the angleof light impingement is also given in FIG. 15.

FIG. 15 illustrates a 1:1 switching with an asymetrical mask withina=Mean Angle of Incident Light and D=Distance from Reticle to mirror.When the reflective width W of the mirror is infinitely small, thethreshold switching is 50:50.

Therefore, when the shadowed width, ‘S’ is equal to 0.5 W, then theeffective reduction in W from both ends will have the effect of making‘W’ infinitely small.

This occurs when S=D.tan(a) and W=2S, so that: W=2D.tan(a) for 1:1switching.

Thus, the true value of this approach is that a set threshold signalstrength for switching can be established for a particular mask/reticleratio, based on the system-imposed variables of an optical fibersNumerical Aperture and the specifics of the detector system. Once, set,however, further variations in the specific fiber signal strength willnot have any effect on the switching ratios.

The change in the threshold signal level switching is shown in FIG. 18,where the original orthogonal light-derived signal is narrowed to thedashed signal profile so that the threshold ‘one’ internal 58 is equalto the threshold ‘zero’ interval 60.

Referring to FIGS. 19 and 19A, there is shown a tensioner/sensorassembly, generally indicated by the reference numeral 62 for a dronecable 61. The assembly 62 includes an optical sensor 12 and a fixedpulley 63 which is fixed rotatably mounted on an axle 72 which is fixedto a mounting bracket 64 attached to a bridge deck or other structure tobe monitored. The housing portion 23 of the sensor is also mounted onthe angle 72. The probe portion 21 of the sensor 12 is rotatably mountedon an angle 73. One end of the axle 73 is fixed to a floating pulley 65.The other end of the axle 73 extends into a vertical slot in themounting bracket 64 for guiding the axle 73 as the floating pulley 65moves toward and away from the housing portion 23 of the sensor 12 asindicated by the double headed arrow 77 in FIG. 19A. The probe portion21 of the optical sensor 12 is fixed outwardly by an internal spring inthe housing portion 23 of the sensor. The outwardly biasing sensor 12 isused in the examples of the present application. However, in someapplications of the invention, an inwardly biased sensor may be used ora sensor which does not have a bias. The drone cable 61 is wrapped atleast once around the pulleys 63 and 65. An optical fiber 66 or fibers66 extend from the optical fiber cable 67 which is deployed next to thedrone cable 61.

The floating pulley 65 is compelled by the internally-sprung encoder tomove away from the fixed pulley 63 to accommodate any slack in the dronecable 61.

Any such movement is registered by the digital mask in the encoder viathe interrogating optical fibers.

In the event that larger cable length changes are anticipated than canbe accommodated by the travel of the encoder, multiple pulley sheavescan be employed to demagnify the cable's travel. Any such adjustmentwould be readily compensated for in the computer software.

The optical sensor is preferably enclosed in a protective box or theequivalent in order to safeguard the sensor from environmental hazards.

The Encoder body is internally spring loaded to the maximum extensionpossible. Referring to FIG. 19A, certain movement-induced compressiveforces will overcome the spring loading and cause the reticle Quadraturestrip 68 to translate past the mask strip 69 and the interrogatingoptical fiber connector at point 71 whose entrained fiber will beemitting a constant high bit-rate stream of pulses. This output will bereflected back into the same fiber for the return trip to the detectorso along as the openings of the reticle strip 68 coincide with thereflective strips of the mask strips 69.

DRONE CABLE SUPPORT SYSTEM

One of the prime features of the distributed fiber cable sensor systemhas been its dual function as both the conduit for the sensor conductorsand the actuator for sensor movement via strategic sensor placement. Insome instances where extreme distances exist between the twointerrogation anchors, in span deflection measurement for example, theremay be problems of accumulated cable weight. Here, we are referring tothe catenary effect of a cable stretched between two mutually distantpoints. Any change in separation between those points should translatedirectly into an equivalent change in the interposed encoder sensor.When the fiber cable is deployed over long distances, there is the riskthat the cables own tendency to catenary under its own weight willnullify its ability to react to the extrinsic actuating agent, in thisexample the downward deflection of a bridge deck under load. The obviouspalliative is to create an overwhelming tensile stress on the cablethrough imposition of a massive expansion spring in the cable. However,a vicious circle is created where the cable must be bulked up to survivethese tensile forces which only makes the cable heavier and more proneto sagging under its own weight, raising the prospect of furtherstrengthening and concomitant weight increase. In order to avoid thiscircular problem, a scheme has been derived which side-steps thisproblem.

THE DRONE CABLE SOLUTION

Many embodiments of the distributed cable system of the presentinvention employ the fiber cable and drone cable for the monitoredstructural segment. The goal of this improvement is to show that thebasic principal of the system may be preserved while adding a series ofapplication-specific distance-registering around fiber drone cableswhose only function is to accurately and swiftly follow the relevantdistance changes.

As shown in FIGS. 19 and 19A, tensioned drone cable is wrapped at leastonce around the two pulley assemblage which comprises the tensionerportion of the optical sensor. The floating pulley is compelled by theinternal spring of the sensor to move away from the fixed pulley toaccommodate any slack in the drone cables caused by movement of themonitored bridge section. Any such movement is registered by the digitalmask in the sensor via the interrogating optical fibers. This way,otherwise separately supported and deployed optical fiber cable enjoysthe ability to vicariously monitor the drone cable's movement withoutthe onus of having to sustain the inter-anchor span stresses. FIG. 21shows the system monitoring for bridge deck deflection. Here, the fibercable monitors the drone cable's movement due to the deflection ofdeck-attached deflectors, which exaggerate the bridge deck motion andthus the drone cable motion, encoder translation, and optical signaltransmission.

DECK DEFLECTION AND PIER TILT MONITORING

Referring to FIGS. 20 and 20A, the bridge deck deflection and pier tiltmonitoring system of the present invention is generally indicated by thereference numeral 70. The system 70 includes an optical sensor 12applied to the bridge deck 74 which is supported between a pair of piers76. The bridge deck motion to be detected is indicated by the arrows 75.A drone cable 78 is located below the bridge deck 74 and extends betweenthe piers 76. Drone cable deflectors 80 are fixed to the underside ofthe bridge deck 74 for maintaining the drone cable 78 spaced from theunderside of the bridge deck. The optical sensor 12 is operativelyconnected by the drone cable 78 and is supported from the underside ofthe bridge deck 74 by one of the deflectors 80, see FIG. 20 inparticular. An optical fiber cable 82 is loosely supported on the bridgedeck 74 and is attached to the deflector 80. An optical fiber 84 isbroken out from the fiber cable 82 and is operatively connected to theoptical sensor. The drone cable 78 is preferably made of an aramid fibersuch as Kevlar®. The advantage of using aramid fiber cable as acensoring component is that it is very flexible but inextensible and hasvirtually zero thermal expansion. Further, it is extremely strong andhazard-resistant, particularly when sheathed by anenvironmentally-protective outer jacket, and is also light in weight.Thus, it lends itself perfectly to deployment as a remote sensingcomponent on bridges and large structures alike. This modification in noway detracts from the original premise of the distributed structuralmonitoring system. Indeed, it extends the application of it through theuse of application-specific drone cables whose sole task is to createthe conditions for an otherwise environment-isolated fiber cable toaddress in its conventional and intended fashion.

It will permit the monitoring of many fiber cable-hazardous environmentsby deploying the appropriate drone cables between the hazardouslocations and the fiber cable-benign sensing area. Obvious examplesinclude high and low temperature, chemicals, nuclear radiation, underwater, and many more. Applications that use the drone cables includedeck deflection and pier tilt sensing, pier scour detection, pilemovement monitoring, wind velocity determination, pothole detection andtraffic monitoring, and building movement and fire detection.

As shown in FIGS. 20 and 20A, the drone cable 78 is artificially heldaway from the underside of the bridge deck 74 under consideration by oneor more of the deflectors. These are interposed between the deck's lowersurface and the drone cable 78 to obviate friction and its attendantabrasion, and also to artificially create a space for sensor deployment.It is possible, although not mandatory, for the deflectors to offerenough lateral flexibility to accommodate the sideways vector motion ofthe drone cable, the natural outcome of the deck's up-and-down motion,without the need for slidable means between the deflectors and the dronecable which would be otherwise necessary.

The lateral movement of a deflector whose base is fixed to the dronecable offers an alternative or even additional site for the location ofa fiber cable and encoder sensor. In this instance, the drone cablecould merely perform the inevitable task of accommodating the stretchingand shortening effects on the drone cable of the deck's downward andupward movements.

As the deflector 80 flexes laterally under the impetus of the deck'svertical motion, the optical fiber cable 78 anchored to the deflectorwill respond and actuate the associated sensor.

FIG. 21 shows a combination deck deflection—pier tilt monitor. Forsimplicity, the fiber cable (which would run along the length of thebridge deck) and optical fibers leading to the encoders are not shown.Bridge deck deflection is indicated by arrow 86 and pier tilt isindicated by arrow 88.

Any movement within the sensor 12 of the bridge deck 74 or piers 76would cause a change in Drone cable length, encoder movement within thesensor 12, and thus optical fiber signal transmission. And with movementsensing up to 60 cm, abnormal deck deflection and pier tilt could bedetected and possible structural damage and catastrophic failureaverted.

Pier tilting can also be monitored by deploying a tiltmeter on the pier.A tiltmeter is a device for detecting and measuring any change inangular attitude of a member to which the tiltmeter is attached,Referring to FIGS. 22A and 22B, an encoder-based tiltmeter of thepresent invention is generally indicated by the reference numeral 54.Tilt meter 54 includes an encoder wheel 55 rotatably mounted on a shaft81 which is fixed to a housing 79. The wheel 55 has a mask with a radialencoder pattern and is disposed to a set position by means of a weight57 attached to its extremity in such a way so as not to interfere withthe encoder wheel rotation within the housing 79. A reticle whichcorresponds to the encoder wheel's mask is attached to the tiltmeterhousing 79. The tiltmeter 54 includes means for interrogating therelative translation of the mask and the reticle. Such means may be aconventional LED-photo diode pair, as employed in conventional electrooptic encoders, or else optical fibers in either reflective ortransmissive deployment geometries.

The mask and reticle layouts may be conventional or else according tothat which has been described in connection with FIGS. 6-18. In thepreferred embodiment on interrogating optical fiber 83 is located on amounting bracket 85 which is fixed to the housing 79.

FUNCTION

The tiltmeter housing 79 is firmly attached to a relevant portion of thestructure whose incipient change in angular disposition would be ofimportance. The interrogating electro optic or optical system willregister the initial status through recording the relative locations ofthe reticle and mask. Upon a change in structure angularity, the housingwill tilt, taking with it the reticle assembly. The encoder wheel withits mask will not change angularity, however, due to the weight attachedto its lower extremity and acted upon solely by gravity. The resultingrelative displacements of the reticle and mask will thereforequantifyably indicate the angular change in structural status.

BRIDGE PIER SCOUR DETECTION

It has long been known that one of the principal reasons for bridgecollapse is scour, the erosion of the substrata beneath river-spanningbridge support piers. Monitoring this has prove to be so difficult thatcurrent protocol calls for suspect bridges to be visually examined everyfive years using frogmen to inspect the submerged portion of such piers.It is our proposition that the distributed fiber and drone cable systemof the present invention can be configured such that even smallmovements arising from the leaning of a compromised pier can be detectedand isolated early enough in the process that compensatory actions maybe taken before the structure reaches the point of catastrophic failure.

INFERENTIAL MEASUREMENTS

The encoder sensor can be calibrated to monitor movements as small asfive microns, or one-third of the width of human hair. By intelligentlydeploying a series of distributed sensors, the system is ableconceptually to detect minute relative movement shifts of the variousbridge components.

The principal here is simple: the first structural effect of scouring,which is the washing away of supporting strata beneath bridge piers, isthe movement of those piers in response. When this movement occurs, itcauses the vertically deployed drone cables to change length. Thesechanges are readily accommodated through sensor motion to detect suchmovement and to report it.

It should be pointed out also that the optical sensors which are locatedto span every critical member junction will monitor any localizedmovements, and will also almost certainly detect sympathetic movements.All of these inputs will be available to the computer data base in realtime and accessible for correspondingly real time analysis andmanipulation.

This illustrates that a major strength of the present system, i.e., isits ability to gather many often disparate data allowing them to becross-correlated instantaneously.

DIRECT MEASUREMENTS

While the foregoing describes the detection of an underwater problemthrough above wage monitoring, there is a strong argument for a moredirect approach whereby the fiber cable system itself is deployed in thelocality of suspected scour. Here, however, the logistics and necessarycharacteristics of the fiber cable system are rather different from thestandard deployment addressed to date. In the first place, there is thequestions of cable tie-off placement especially in the context of thetangible drag and disturbance of water flow and possible ice formation.Both of these agents could easily disrupt the system and cause it tobroadcast phantom alerts. The location and nature of the suspectedproblem areas must first be determined. The problem areas will mostlikely involve the erosion of pier foundations, often where remedialactions are either contemplated, under installation, or else already inplace. Any one of these three scenarios will permit an interactivedesign opportunity because the most critical erosion candidate areaswill have already been identified. It will be necessary to have beenappraised by the experts of critical parameters such as the location,the allowed erosion depth, the allowed sub-pier incursion distance, etc.This information will offer crucial knowledge of where and what tosense.

Small movement monitoring in an unpredictable and hostile environment,submerged in a potentially fast-flowing and foreign body-laden stream ofwater is the challenge. The sensing system must be isolated as far aspossible from the spurious effects of the environment.

Referring to FIG. 22, the placement of the sensor will devolve from theexperts' analyses, and will preferably have the form of under-wateraramid fiber drone cable extensions. The sensors are sheathed within arugged protective conduit to protect the sensors from floating detritusor even legitimate maritime traffic which could equally disrupt thedetection system's integrity. The upper end of a drone cable 90 isattached to an optical sensor 12 at some point above the water on thebridge pier 76. After descending underwater, the cables will follow thelength of the bridge pier to the floor 94 of the waterway. Here, thecable 90 is tied off to a weighted concrete block 96 that rests on thewaterway floor. When the motion of the underlying substrata is largeenough to move the concrete blocks, the corresponding drone cable andencoder movements will lead to signal changes in the interrogatingoptical fibers 97 from the optical fiber cable 98. Thus, both theoccurrence and location of substrata erosion will be detected by thesystem. With the integrity of the bridge structure continuouslymonitored, sudden, drastic changes, in the stability of the substrata,such as during or after a major flood, can be evaluated, and if the pierscour damage is deemed severe, alarms can be posted and bridgeauthorities notified immediately.

The approach described above addresses the placement of the sensorsabove the water level but attached via drone cables to the submergedsites. Further, it creates a hybrid analog-digital sensing modalitywhere real trouble is indicated by the gross movement associated with aconcrete block whose large movements will indicate the erosion of asection of river bed, but with the expectation that smaller detectedprecursor motions will most likely forecast the digital catastrophicsensor failure. It also addresses the hostile environment throughkeeping all fiber optical components above the water in the morecontrolled environment, but placing only rugged cable and concreteblocks, effectively, under the water.

Bridge pier scour, which is the erosion of the substrata beneathwaterway-spanning support piers, is recognized as one of the principalreasons for bridge collapse. Thus, methods of averting pier scourdisasters by strengthening the pier structure with deep foundationattachments have been the subject of much recent research. These deepfoundation elements are known as piles (or micropiles when they aresmall diameter structures), and they have been used typically as a loadtransfer connection from the bottom of the bridge pier to competentsubsurface strata. Piles have also been used in other water-spanning andlanded structures alike, particularly micropiles, which have been usedto strengthen historic buildings across the world. But like bridge pierscour detection, monitoring these structures has proven difficult sincethe piles may extend well below the floor of the waterway grand surface.It is our proposition that the fiber cable system of the presentinvention can be configured such that even small movements of the pilescan be detected and isolated early enough in the process before theonset of erosion, loosening of the piles from the stable substrata, andpossible catastrophic failure of the piles and/or the supportedstructure.

The first structural effect of scouring on bridge piers is the movementof those piers in response to the now unstable adjacent substrata.Similarly, as the soil moves or begins to erode around the pile, it toowill move in response, and abnormally large translations will indicatethe overall loosening of the pile from the stable substrata, and thusthe imminent likelihood that pile failure would occur. Referring to FIG.23, for each bridge pier 76, vertically deployed, environmentallyprotected drone cables 100 are attached to optical sensors 12 at the topof the pier 76. While the sensors 12 and the interrogating optical fibersensor cable 102 would remain well above the water, only the protectedsheathed drone cables enter the potentially hazardous environment. Asthe drone cables 100 descend below the pile cap, they are looped aroundthe top of the piles 104 in the river bed 106, each drone securelyattached to a pile or a group of piles. Since they, like the otherdrones, are made of movement-sensitive arramed material, the slightestmovements of the piles would be transmitted by the drones to theattached encoder sensors. the change in the incoming signal by theencoder would then be readily perceived by the outgoing fiber path,which would report such data immediately. All of these inputs will beavailable to the computer data base in real time and accessible forcorrespondingly real time analysis and manipulation.

This illustrated a major strength of the technique of the presentinvention, its ability to gather many often disparate data allowing themto be cross-correlated instantaneously. Thus, the entire pile systemcould be monitored full-time. And with a movement resolution as small asfive microns, the state of the piles can be tracked and possible failurepredicted, whether attached to bridge piers, oil rigs, historicbuildings, or other structures with foundation support systems.

BRIDGE TEMPERATURE SENSOR SYSTEM

Referring to FIG. 24, an optical sensor 12 is tensioned between azero-expansion fiber optical jumper cable 108 and a known thermalexpansion calibrated rod 110 attached to a substantial portion of thebridge, such as a pier 107, away from direct sunlight.

The calibrated rod expands and contracts with the changes in temperatureand causes the optical sensor 12 to accommodate any length changesresulting therefrom. The fiber cable jumper 108 (i.e. the interrogatingoptical fibers) carries the displacement information back to the trunkcable 112 and thence to the modem.

WIND VELOCITY SENSOR SYSTEM

Referring to FIG. 25, an arramed cable 114 is attached to awind-sensitive sensor or material 116, and is strung from the bottom ofthe bridge deck 118 to the pier wall, where the cable is connected to anoptical sensor 12. Wind speed and direction changes will cause the dronecable to move in response, and such changes are recorded instantaneouslyin the interrogating pulse signal which is carried back to the modem viathe fiber cable jumper 120 and the trunk optical cable 122.

POTHOLE DETECTION AND TRAFFIC FLOW MONITORING

Referring to FIG. 26, two drone cables 124 and 126 are stretched acrossthe bridge deck surface 128 a set distance apart. They are each attachedto a dedicated optical sensor 12 hooked into the fiber opticinterrogation harness. Each optical sensor 12 is connected to an opticalfiber 130 from a trunk cable 132 which is part of the fiber opticinterrogation harness. Thus, any vehicle 134 passing over the deck willtrigger a response in the two sensors 12 which will transmit theinformation back to the modem. Alternatively, sensors placed at each endof a deck section to monitor deck motion and vibration will be actuatedby the passage of proximal traffic. In FIG. 27A, the sensors 12 arepositioned at seams and 52 in the bridge deck. A pothole to be detectedis indicated at TP at the surface 128 in the span 127. A vehicle passingover the seams S1 and S2 will trigger a response in the two sensors 12.

When potholes appear in the roadbed, exaggerated pounding will accrue tothe deck which will both eventually cause damage and, more immediately,cause the deck vibration sensors to see large amplitude excursions thanwould be expected with a pothole-free roadbed for the same vehicleconditions. The problem has always been to know the type and velocity ofthe vehicles involved with pothole interactions in order to quantify thevibrational effect.

With a full-time monitoring system there is a way to do this just aslong as there is sometime during the twenty-four period when only asolitary vehicle is passing over the bridge. Better yet, the resultswould be far more indicative if the vehicle type and velocity wereknown.

Using the real-time monitoring capability of the present system and theappropriate computer algorithm, the computer will recognize a solitaryvehicle, compute its velocity from both the time taken from point A toPoint B and the time dwell of the tires on the drone cables, andrecognize a tractor trailer, for example, from its distinctive wheelsequence signature. When the solitary tractor trailer conditions arerecognized, the computer will record the deck vibration amplitude dataand normalize them for the measured A-to-B velocity. These data will bestored and continuously trend-analyzed to see if some extrinsic factor,such as a pothole, or even ice build-up, is causing an anomalously largevibrational deck affect. The pothole sensing algorithm is illustrated inFIG. 27B.

TRAFFIC FLOW MONITORING

As illustrated in FIG. 27, a solitary vehicle's velocity can bedetermined from both the time taken from point A to point B and the timedwell of the tires on the drone cables. If traffic flow is light, thensingle vehicles traveling over a bridge or a certain segment will bemore common, and their speeds thus computed with ease, so long as thedistinctive wheel sequence signature is recognized. If, however, trafficis congested or even at a standstill on the bridge, the great amount oftime that tires spend on the drone cables (since they will be moving atzero or near-zero velocity in heavy traffic) will be translatedinstantaneously through the optical sensor 12, with the new displacementinformation carried back to the modem via the fiber cable. Thus, as soonas traffic jams start to form on a monitored bridge or another installedsection of roadway, TV and radio station traffic patrols can be notifiedimmediately, and the general public alerted to these traffic problemssooner than with modern on-site helicopter monitoring practices.

CORROSION MONITORING

The system of the present invention integrates all factors leading torebar corrosion by placing a sacrificial rod in contact with theconcrete matrix under investigation and exploits two distinct aspects ofthis controlled corrosion.

ROD INCREASE SEQUENCE

Referring to FIGS. 30 and 31, a rebar-like metal rod 137 in contact witha matrix 139 will corrode at its end which is in contact with the matrixto produce a corrosion product such as rust which sloughs off, aindicated by the reference numeral 141. This causes the metal rod 137 todecrease in size over time as shown in FIGS. 30 and 31.

Referring to FIGS. 28 and 28A a rebar-like metal rod 136 whose distaltip is corroding in contact with the concrete matrix 138 will follow anError Function (erf) expansion length increase. This initially rapid andthen progressively slowing length change is due to the increasingthickness of the corrosion layer, which forms with approximatelyfourteen times the volume of the metal consumed. This corrosion product140 forms a barrier which progressively retards the reaction-criticalion counter-diffusion. This modality therefore promises a relativelyrapid initial indication of rebar corrosion, but is of questionablefuture tracking value. It is designated the Corrosion Onset Sensor, COS.

ROD SHRINKAGE SEQUENCE

The concrete deck corrosion monitoring system of the present inventionis a retrofit-compatible concept with potential application to virtuallyevery pre-existing or new concrete structure. The Federal HighwayAdministration had identified over 170,000 US bridges in need of somesubstantial repair, many of which were due to deck rebar corrosion. Oneof the nagging problems with such structures as bridges, high-riseparking lots and large building has been the absence of precise andquantifiable information regarding the corrosion state of the rebars andthe corresponding need for counter-corrosion measures.

This approach offers an auto-integration of corrosion propensity if therebar is corroding at a certain location and at a fixed depth, then, itis likely that its neighboring rebars are suffering similar fates. Ifthere is a great deal of variability in corrosion potential within a setstructure, then many of the intrinsically simple and incipiently lowcost direct visualization sensors may be interspersed with a few numberof the full-time and therefore more expensive sensors.

THE LOW COST, DIRECT VISUALIZATION BRIDGE DECK CORROSION SENSOR

Referring to FIGS. 32 and 33, the system relies on the sacrificial minirebar rod concept for a remote sensor. The indicator is a bent resilientsteel lath 148 held into its bent posture by one or two mini rebar rods150. When the rods 150 begin to corrode, the lath progressively opens upas shown in FIGS. 33 and 35. A glass observation port 152 is locatedabove the borehole 154. The borehole 154 in the concrete matrix 156 isfilled with a silicone filler between the lath 148 and sides of theborehole.

Referring to FIGS. 34 and 35, a resilient cylindrical stainless steeltube 158 is inserted in a borehole 160 in the concrete matrix 162. Thetube is squeezed into an elliptical shape by mini rebar rods 164. As therods 164 corrode, the tube 158 returns to its normal cylindrical shapeas shown in FIG. 35. Spring indicators 166 are located at the tip of thetube 158 to provide a visual indication of movement of the tube 164 as aresult of corrosion of the rods 164. The tube 158 is surrounded by asilicone filler 168.

ALTERNATE CONCEPT

Referring to FIGS. 36 and 37, a right cylindrical resilient stainlesssteel tube 170 is employed in a borehole 174 in the concrete matrix 175which has a single mini rebar rod 172 attached at a definite level abovethe base of the tube. This will become an interference fit in theborehole 174 such that the tube 170 is distorted into an ellipticalshape at the rebar's location. This distortion provides the springimpetus maintaining the rebar 172 in contact with the borehole wall, aswell as assuring that the rebar will progressively penetrate anycorrosion product at the point of corrosion 173.

Any change in ellipticity resulting from corrosion-induced rodshortening will be reflected and magnified by a first degree lever 176attached to the inside of the stainless steel tube at the rebar's anchorpoint. Acting through a simple fulcrum at a set distance proximal to therebar, the lever's opposite extremity will terminate just below theplane of the bridge deck's surface.

The lever 176 is pivoted at 178 to a cross bar 180 which is fixed to theinner surface of the tube 170. The space between the tube 170 and theinner surface of the borehole which is occupied by the rebar 172 isfilled with a silicone filler 182. A transparent cap 184 is located atthe top of the borehole 174 for visual observation of the change ofposition of the top of the lever 176 which is indicative of corrosion ofthe rebar. The cap 184 is provided with a scale 186.

Referring to FIGS. 38 and 39, a right cylindrical stainless steel tube185 is employed which has a single rebar mini rod 187 attached at adefinite level above the base of the tube. This will become aninterference fit in the borehole such that the tube is distorted into anelliptical shape at the rebar's location. This distortion provides thespring impetus maintaining the rebar in contact with the borehole wall,as well as assuring that the rebar will progressively penetrate anycorrosion product. The tube distortion may be used to actuate an encoderor other remote monitoring device.

Any change in ellipticity resulting from corrosion-induced rodshortening will be reflected and magnified by a first degree lever 183attached to the inside of the stainless steel tube 185 at the rebar'sanchor point. Acting through a simple fulcrum at a set distance proximalto the rebar, the lever's opposite extremity will terminate just belowthe plane of the deck's surface.

Referring to FIGS. 40 and 41, there is illustrated two embodiments of asurface mounted probe assembly for monitoring the corrosion of concrete.Corroded concrete is referred to in the industry as “punky concrete”.

The first embodiment of FIG. 40 is generally indicated by the referencenumeral 191 and includes a cylindrical housing 192 which has acylindrical bore 197 and a bottom outer flange 193. The flange 193enables the assembly 191 to be mounted to the upper surface 194 of aconcrete structure 195 by means of fasteners 196. A sealant 189 islocated between the flange 193 and the surface 194 of the concrete. Theupper end of a probe 198 is fixed to a cylindrical weighted piston head199 which is slidably mounted in the bore 197. The lower end of theprobe is biased into engagement with the surface 194 by the piston head199. The downward biasing of the probe 198 could also be provided by aspring. An elastomeric sealant 200 is located between the probe 198 andthe inside surface of the bore 197. The housing portion of an opticalsensor 12 is fixed to the housing 192 of the probe assembly by a housinganchor 201. The probe portion of the sensor 12 is biased downwardlyagainst the upper end of the piston head 199. The sensor 12 isoperatively connected to the fiber optic cable by optical fibers 202.Corrosion or softening at the surface 194 of the concrete will cause theprobe 198 to be moved downwardly by the weight of the piston head 199.This movement of the probe 198 causes the probe portion of the sensor 12to move downwardly relative to the housing portion of the sensor,thereby producing an optical signal which is indicative of the softeningcondition of the concrete.

The second concrete monitoring assembly illustrated in FIG. 41 isgenerally indicated by the reference 203. Assembly 203 is identical toassembly 191 except that it does not include a sensor 12. The elementsof assembly 203 which are identical to assembly 191 are identified bythe same reference numerals. The probe assembly 203 includes a removabletop cover 204 which is mounted on the cylindrical housing 192 above thepiston head 199. A micrometer 205 is mounted in the cover 204. Themicrometer includes a stilus 206 which extends below the cover forengaging the upper surface of the piston head 199 and a gauge 207located above the cover. Any downward movement of the probe 198resulting from corrosion or softening of the concrete can be readdirectly from the gauge 207.

FATIGUE FUSE

A fatigue fuse is a pre-weakened metal member which is attached to astructure which may experience fatigue failure problems. The fuse memberexperiences the strain history of the structure and fractures at itspre-weakening notch site after a known accrual of fatigue. Fuses aregenerally made in sets of four with a sequenced fracture profile.

Referring to FIGS. 42-45, the fatigue fuse monitoring system of thepresent invention includes a reflective grid generally indicated by thereference numeral 230 in FIG. 42 and a transmissive mask, generallyindicated by the reference numeral 232, in FIG. 43. The reflective grid230 includes a plurality of parallel spaced tines 234 extending from abase 236. Each tine 234 has a small notch 238 which functions as afatigue initiator. The transmissive mask of FIG. 43 has a plurality ofspaced fiber optic connector locations 240 which correspond to thespacing of the tines 234 as depicted in FIG. 44 which shows the grid 230overlaying the mask 232.

The assembled fatigue fuse assembly is generally indicated by thereference numeral 242 in FIG. 45. The reflective grid portion 230 of thefatigue fuse is attached to a substrate 245 being monitored by anadhesive 247. The base 236 of the transmissive mask portion of thefatigue fuse is fixed to the reflective grid portion of the fuse at 249.

The reflective grid 234 has a periodicity of 30 microns, which matchesthe fuse movement once the fatigue-initiated crack has propagated fullyacross the affected fuse leg or tine 234. When illuminated through theequivalently set up Transmission Mask , as shown in FIG. 43, thereflected light signal will change in magnitude by comparison with theother unaffected fuses.

STEREOGRAPHIC REPRESENTATION OF MASK OVER FUSE

Fatigue monitoring, a development of Materiall Technology, Inc.,“MaTech”, is a remarkable achievement because it comprises theimperative two-step process which first evaluates the fatigue alreadypresent in the metal member and then goes on to continuously monitorthat same member on a quantitative basis from the freshly establishedbaseline.

The first phase, which is to diagnose the accumulated fatigue, is a onetime hands-on procedure which, appropriately, yields an EKG-likewaveform output which is interpreted. The result is an assessment of thelevel of the fatigue present in the member at that instant, to anaccuracy of about 15 percent.

The second part of this fatigue equation is the Fatigue Fuse. Thefour-tined comb as shown in FIG. 42 is made from the same compositionalloy as the member under investigation. The Fuse assemblage is cementedto the member at its extremities and therefore compelled to faithfullyexperience the very same surface stresses as the member itself from thattime forward.

Each tine is preconditioned to fail at a different, say, 10 percentageincrement of additionally accumulated fatigue. Thus, if the member hadbeen diagnosed as evidencing a 40 percent fatigue level when the FatigueFuse was attached, each fuse failure will signal the additional 10percent increments in a progressive fail-soft and remediable manner. Thecable interrogation system has been configured to detect the minute 40micron fuse failure cracks as they occur.

FUSE OPTICAL INTERROGATION AND STRAIN GAUGE ATTRIBUTES

The optically-interrogated Fatigue Fuses can provide exactly the type ofdata currently gathered in conventional strain gauge monitoring system,such as Lockheed Martin's IHUMS inferential fatigue monitoring approach,at least up to the point where they actually fatigue to fracturefailure. At failure, of course, they are indicating in the mostassertive manner that a critical accumulated strain datum of fatigue hasbeen reached, regardless of what any inference-based software isindicating. This offers the best of both worlds.

With the optically-interrogated Fuse, one has an accelerometer whoseconstantly-monitored vibration signature lends itself directly to FFTanalysis and the fundamental frequency information available gleanedtherefrom. In addition, it provides a digital and therefore absolutevalue of any vibration amplitude excursions.

Optimally then, the Fuse System offers all of the standard strain gaugeinformation plus the reassurance of the actual fuse fracture event andan adjacent and greater longevity fuse ready to assume the task ofgenerating ongoing accelerometer data after the first fuse has failed.

The attraction of this scenario is that the same fatigue fuse which willeventually fracture will provide sub-critical strain accumulation dataup to the point when it actually does fracture. These data will beamenable to fatigue-predictive manipulation. As such, this non-invasiveand auto-generated data source should greatly assist in joint fatiguemodeling and aid considerably in design refinement leading to a morebasic understanding of the various uniform and jointed structure fatiguephenomena. An example of fuse testing results is shown in FIG. 46.

FATIGUE FUSES: REPRODUCTIBILITY OF TEST DATA

More than 50 precision Fatigue Fuses with 200 notched Tines have beenFatigue Tested

Most tests employed long variable stress sequences to simulate realisticconditions

Variables included: Fuse Material, Adhesive, Size of bond area, Shape ofFatigue notch, Thickness of fuse

Multiple replicates were employed

Scatter in results is relatively small

Fatigue Tines fail in the programmed sequence, indicating theprogressive fatigue experienced by the substrate

BUILDING MONITORING

The present system incorporates many conductors, most of them opticalfiber. In the inter-high-rise building movement monitoring and firedetection scenarios, it is very desirable from a code and ease ofimplementation standpoint that there only be optical fibers or aramidfiber components since both are non-conductive and therefore present noelectrical hazard. The Building Movement and Fire Detection System wasdeveloped using a combination of a digital encoder sensor-fiber opticinterrogation harness in combination with a series of aramid dronecables. Using an aramid cable as the linkage medium between criticalstructural building members and the encoders, and the fiber cableharness as the sensor interrogation means, it is possible to create awhole-building network of sensors which will respond to building motionsin any of the x, y, z coordinate directions.

The advantage of the aramid cable is that it is very flexible butinextensible and with virtually zero thermal expansion. Further, it isextremely strong and light in weight. Thus, it perfectly lends itself todeployment as a web network throughout large structures, typicallyplaced above dropped ceiling and well out of the way.

Any motion in the building which causes the encoder-tensioned aramidcables to elongate or to shorten, such as during an earthquake, willimmediately translate into encoder motion, in turn instantaneouslytransmitted back to the base computer monitoring system for analysis andresponse. In the event of a fire, the aramid drone cables' deployment insprinkler-grade wax captured convolutions, see FIG. 47, will triggersite-specific sensor changes which may be computer recognized asdistinct from earthquake events by their specificity. This will simplycome about from the aramid cables' convolution release as the ambienttemperature rises to soften the wax adhesive.

The drone aramid cables 244 may also be deployed using a simple pulleysystem generally indicated by arrow 246, as shown in FIG. 47, whichincreases the area coverage for fire detection. The pulleys 248 attachedto the east-west walls only in this representation so that any changesin the drone cable will be due either to wax melting in one or more thewax-enclosed S-shaped cable sections 250 else specific relativedisplacement of the east-west walls. Further, the displacement will bedirectly proportional to the wall movement, albeit reduced by a pulleyreduction factor. Given the high resolution capability of the DigitalSensor, this “pulley-induced demagnification effect” is not a problem.The drone cable 244 is connected to a sensor 12. An optical fiber 252from a fiber optic cable 254 is connected to the sensor 12.

BUILDINGS AND BRIDGES

Bailey Bridge integrity, and other perhaps more permanent criticalmilitary structures would be excellent candidates for the system of thepresent invention, where the ease of uncoiling and deploying thistwo-way-reflective and therefore essentially one-ended “rope withornaments attached” is readily adapted to a wide array of geometries.This system has the ability to instantly report on the “health” ofhigh-rise buildings, post-earthquake. The military theater has much incommon with earthquake-prone terrain from the standpoint of jarringvibrational damage and so may see useful parallels.

NAVAL VESSELS AND AIRCRAFT

Another military fit for the Fatigue Fuse of the present invention isobviously airframes and oil tanker structural monitoring, naval vesselsare obvious candidates. The nonelectric aspect of the fiber optic systemshould play here, in what would be a literally explosive environment, aswell as it does in the equally dangerous milieu of oil freighters. Inaddition, the bolt clamp load monitoring system of the present inventionwill also fit in well here, particularly on some of the newest modelairframes.

TEMPERATURE MONITORING

On at least one shaded portion of the bridge structure which is known tobe extremely rigid, a section of thermally expandable cable will beutilized to deliberately react to changes in temperature. This isbecause several sections of the bridge structure will include metallicmembers which will expand and contract with temperature fluctuations. Inorder to be able to separate these effects from, for example, legitimatestructural shifts as recorded by the relevant motion sensors, thecorrective factor must be known. Referring to FIG. 48, one end of atemperature expanding cable 256 is fixed to an anchor 258 on the bridgestructure 259. The other end of the expanding cable 256 is fixed to theprobe portion 21 of a sensor 12. An aramid fiber cable 260 which doesnot expand. As a result of increases in temperature is fixed to thehousing portion 23 of the sensor 12 to an anchor 262 which is fixed tothe bridge structure 259.

FUNCTION

Referring to FIG. 66, the temperature-related changes of the ExpandingCable will be detected through the accommodating motion within theoptical encoder. These changes will occur according to the laws ofthermal expansion and contraction.

These state that the change in length, L1−L2=dL will be:

dL=L 1×Tc×(T 1−T 2),

where: Tc is the thermal expansion coefficient of the Expanding Cable

T1 is the starting temperature in degrees Celsius, and

T2 is the ending temperature in degrees Celsius

Note that the Drone cable is made of Kevlar which has a zero temperaturecoefficient of expansion.

What is claimed is:
 1. Apparatus for monitoring a structure comprising:(a) a source of power; (b) a transceiver connected to said source ofpower for transmitting and receiving pulses of light; (c) a plurality ofoptical sensors for deployment on said structure, at least one of saidsensors having a first element operatively connected to a first part ofsaid structure and a second element movable relative to said firstelement and operatively connected to a second part of said structure, atleast one position of said second element relative to said first elementbeing a first state of said sensor and at least one position of saidsecond element relative to said first element being a second state ofsaid sensor, each of said sensors being capable of receiving pulses oflight in said first and second states and for transmitting said pulsesof light in said second state; (d) a cable of optical fibers fordevelopment said structure, said cable having at least one plurality ofoptical fibers operatively connected to said transceiver, each of saidsensors being connected to at least one of said optical fibers fortransmitting said optical pulses from said transceiver to said sensorand for returning said optical pulses from said sensor to saidtransceiver when said sensor is in its second state; and (e) the firstand second elements of at least one of said sensors being connected tosaid structure by a drone cable so that changes in a selected conditionof said structure are transmitted to said one sensor through said dronecable, thereby enabling said drone cable to be made of a material whichhas desirable characteristics for physically transmitting changes in theselected condition of said structure and for deployment inenvironmentally hazardous areas and enabling said cable of opticalfibers to passively deploy in environmentally safe areas of thestructure.
 2. Apparatus for monitoring a structure as recited in claim1, wherein said drone cable is made of an aramid fiber.
 3. Apparatus formonitoring a structure as recited in claim 1, wherein said opticalsensor has a first element and a second element which is movablerelative to said first element, said drone cable having a first lengthwhich is connected to a first part of said structure and to said firstelement and a second length which is connected to a second part of saidstructure and to said second element.